kconfig: correct oldconfig for unset choice options

oldconfig currently ignores unset choice options and doesn't ask for them.
Correct the SYMBOL_DEF_USER flag of the choice symbol to be only set if
it's set for all values.

Signed-off-by: Roman Zippel <[e-mail]>
Signed-off-by: Sam Ravnborg <[e-mail]>

scripts/kconfig/confdata.c

···
 		for (e = prop->expr; e; e = e->left.expr)
 			if (e->right.sym->visible != no)
 				flags &= e->right.sym->flags;
-		sym->flags |= flags & SYMBOL_DEF_USER;
+		sym->flags &= flags | ~SYMBOL_DEF_USER;
 	}
 
 	sym_change_count += conf_warnings || conf_unsaved;